The antigen receptor proteinsof vertebrates (antibodies and T-cell receptors)are diverse molecules. By this, we mean that the antigen binding sites have variable amino acid sequences (while the amino acid sequences of the other parts of these molecules are reasonably constant). For example, antibodies produced by two distinct B-cells will differ at their antigen binding sites. This system ensure that when taken together, the full complement of these antigen receptorsin an individual is capable of binding to a wide array of foreign molecules. Further more, if the antigen molecule that is recognised by a specific antibody were to change slightly (e.g. by mutation), then it is likely to be detected by another antibody. Another advantage of this property of the vertebrate adaptive immune system is that it 'anticipates' future encounters with foreign molecules. Since the variable regions of the antigen receptor proteins are all different, then the system is prepared to recognise foreign molecules at later stages in the life of an individual. In evolutionary terms, even if an adult vertebrate were to be infected with a novel pathogen, there is every chance that the pathogen will be recognised and eliminated by the adaptive immune system.

Manyimmunologists believe that invertebrates are not capable of such anticipatory immunity. Invertebrates lack the immunoglobulin-based adaptive immune system of the vertebrates. Those animal rely on immunological systems that are similar to the innate system of the vertebrates. The receptor molecules of the vertebrate innate immune system (called Pattern Recognition Receptors, PRRs) recognise highly conserved features of microbial pathogens (Pathogen Associated Molecular Patterns, PAMPs). Most PAMPs are components of important structural and functional features of the microbes (e.g. cell wall components, DNA, flagella). Since it is unlikely that microbes can exists without such components, it is safe to assume that a system that recognises PAMPs will be very effective in recognising most microbial pathogens.

However, that does not seem to be enough.

Molecular diversity and immunity in invertebrates

Comparative genomics of the invertebrate deutrostomes

Physiological stress and immunity in invertebrates

Re-infection immunity in invertebrates

Anti-viral defense in invertebrates

Sham Nair 2014